The spinal column is a highly complex system of bones and connective tissues that provides support for the body and protects the delicate spinal cord and nerves. The spinal column includes a series of vertebrae stacked one on top of the other, each vertebral body including an anterior portion of relatively weak cancellous bone and a posterior portion of relatively strong cortical bone. Situated between each vertebral body is an intervertebral disc that cushions and dampens compressive forces experienced by the spinal column. A vertebral canal containing the spinal cord and nerves is located behind the vertebral bodies. In spite of the complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction. For example, the kinematics of the spine normally includes flexion, extension, rotation and lateral bending.
There are many types of spinal column disorders including scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis (abnormal backward curvature of the spine, usually in the lumbar spine), spondylolisthesis (forward displacement of one vertebra over another, usually in the lumbar or cervical spine), and other disorders caused by abnormalities, disease, or trauma, such as ruptured or slipped discs, degenerative disc disease, fractured vertebra, and the like. Patients that suffer from such conditions usually experience extreme and debilitating pain, as well as diminished nerve function. These spinal disorders, pathologies, and injuries limit the spine's range of motion, or threaten the critical elements of the nervous system housed within the spinal column.
The treatment of acute and chronic spinal instabilities or deformities of the thoracic, lumbar, and sacral spine has traditionally involved rigid stabilization. For example, arthrodesis, or spine fusion, is one of the most common surgical interventions today. The purpose of fusion or rigid stabilization is the immobilization of a portion of the spine to affect treatment. Rigid stabilization typically includes implantation of a permanent, rigid assembly having metallic rods, plates and the like that secure selective vertebrae relative to each other. Spinal treatment using rigid stabilization, however, does have some disadvantages. For example, it has been shown that spine fusion decreases function by limiting the range of motion for patients in flexion, extension, rotation and lateral bending. Furthermore, it has been shown that spine fusion creates increased stresses and therefore, accelerated degeneration of adjacent non-fused segments. Another disadvantage of fusion is that it is an irreversible procedure.
More recently, dynamic stabilization has been used in spinal treatment procedures. Dynamic stabilization permits enhanced mobility of the spine while also providing sufficient stabilization to effect treatment. One example of a dynamic stabilization system is the Dynesys® system available from Zimmer Spine of Edina, Minn.
Dynamic stabilization systems are designed to more closely mimic natural spine movement, especially in flexion and extension. In many dynamic stabilization systems, however, the systems' rotational axis is positioned away from the normal rotational center of the spine in the posterior direction by a relatively large amount. Such posterior location of the stabilization system alters the natural movement of the spine. Thus, locating the stabilization system more anteriorly may provide an advantage.
Additionally, being able to easily adjust the stiffness characteristic of the system in a simple manner is also desired. Finally, providing a simple mechanism for adjustable distraction between vertebrae is another useful feature of a dynamic stabilization system.
Accordingly, there is a need for an improved dynamic stabilization system and method of using the same that addresses these objectives.